Analysis of phenylbutazone residues in horse tissues with and without enzyme-hydrolysis by LC-MS/MS.

Phenylbutazone (PBZ) is permitted to be used for the treatment of musculoskeletal pain and inflammation in race horses but it is not approved for use in horses destined for human consumption. In a recent study initiated in our laboratory to study the disposition of PBZ and its oxyphenbutazone (OXPBZ) metabolite in equine tissues, we compared the effect of an additional enzymatic hydrolysis step with ß-glucuronidase on the results of the analysis for PBZ without enzymatic hydrolysis. Incurred tissue samples obtained from a female horse dosed with PBZ at 8.8 mg/kg for 3 days and sacrificed 6 days following the last administration were used for this study. Liver, kidney, and muscle tissues were collected, extracted, cleaned up on a silica-based solid-phase extraction (SPE) preceded by a weak-anion exchange SPE and analyzed with our in-house validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for PBZ and OXPBZ. Addition of the hydrolysis step resulted in a significant increase in recovery of both PBZ and OXPBZ residues. © 2016 Her Majesty the Queen in Right of Canada. Drug Testing and Analysis © 2016 John Wiley & Sons, Ltd.

Nonsteroidal anti-inflammatory drug sensitizes Mycobacterium tuberculosis to endogenous and exogenous antimicrobials.

Existing drugs are slow to eradicate Mycobacterium tuberculosis (Mtb) in patients and have failed to control tuberculosis globally. One reason may be that host conditions impair Mtb's replication, reducing its sensitivity to most antiinfectives. We devised a high-throughput screen for compounds that kill Mtb when its replication has been halted by reactive nitrogen intermediates (RNIs), acid, hypoxia, and a fatty acid carbon source. At concentrations routinely achieved in human blood, oxyphenbutazone (OPB), an inexpensive anti-inflammatory drug, was selectively mycobactericidal to nonreplicating (NR) Mtb. Its cidal activity depended on mild acid and was augmented by RNIs and fatty acid. Acid and RNIs fostered OPB's 4-hydroxylation. The resultant 4-butyl-4-hydroxy-1-(4-hydroxyphenyl)-2-phenylpyrazolidine-3,5-dione (4-OH-OPB) killed both replicating and NR Mtb, including Mtb resistant to standard drugs. 4-OH-OPB depleted flavins and formed covalent adducts with N-acetyl-cysteine and mycothiol. 4-OH-OPB killed Mtb synergistically with oxidants and several antituberculosis drugs. Thus, conditions that block Mtb's replication modify OPB and enhance its cidal action. Modified OPB kills both replicating and NR Mtb and sensitizes both to host-derived and medicinal antimycobacterial agents.

Pharmacokinetics of phenylbutazone and its metabolite oxyphenbutazone in miniature donkeys.

OBJECTIVE: To describe the pharmacokinetics of phenylbutazone and oxyphenbutazone after IV administration in miniature donkeys. ANIMALS: 6 clinically normal miniature donkeys. PROCEDURE: Blood samples were collected before and 5, 10, 20, 30, 45, 60, 90, 120, 180, 240, 300, 360, and 480 minutes after IV administration of phenylbutazone (4.4 mg/kg of body weight). Serum was analyzed in triplicate by use of high-performance liquid chromatography for determination of phenylbutazone and oxyphenbutazone concentrations. The serum concentration-time curve for each donkey was analyzed separately to estimate model-independent pharmacokinetic variables. RESULTS: Serum concentrations decreased rapidly after IV administration of phenylbutazone, and they reached undetectable concentrations within 4 hours. Values for mean residence time ranged from 0.5 to 3.0 hours (median, 1.1 hour), whereas total body clearance ranged from 4.2 to 7.5 ml/kg/min (mean, 5.8 ml/kg/min). Oxyphenbutazone appeared rapidly in the serum; time to peak concentration ranged from 13 to 41 minutes (mean, 26.4 minutes), and peak concentration in serum ranged from 2.8 to 4.0 mg/ml (mean, 3.5 microg/ml). CONCLUSION AND CLINICAL RELEVANCE: Clearance of phenylbutazone in miniature donkeys after injection of a single dose (4.4 mg/kg, IV) is rapid. Compared with horses, miniature donkeys may require more frequent administration of phenylbutazone to achieve therapeutic efficacy.

Disposition and tolerance of suxibuzone in horses.

Suxibuzone (SBZ), a nonsteroidal anti-inflammatory drug, was administered to 6 horses at a dose rate of 7.5 mg/kg bwt by intravenous (i.v.) route. Plasma and synovial fluid concentrations of suxibuzone and its main active metabolites, phenylbutazone (PBZ) and oxyphenbutazone (OPBZ), were measured simultaneously by a sensitive and specific high-performance liquid chromatographic method. The pharmacokinetic parameters were determined by noncompartmental analysis. Plasma SBZ concentrations rapidly decreased and were not detectable beyond 20 min after treatment. The parent drug was not detected in any synovial fluid samples. Average maximum plasma concentrations of PBZ (16.43 microg/ml) and OPBZ (2.37 microg/ml) were attained at 0.76 and 7.17 h, respectively. The mean residence time (MRT) of PBZ was 6.96 h in plasma. Oxyphenbutazone plasma concentrations were below those reached by phenylbutazone during the first 12 h after suxibuzone administration, even though its values were detectable for at least 24 h (MRT = 10.65 h). Plasma concentrations of PBZ and OPBZ exceeding EC50 and IC50 of TXB2 and PGE2 were reached by at least 12 h. Synovial fluid concentrations of PBZ and OPBZ were 2.87+/-0.37 microg/ml and 0.97+/-0.08 microg/ml at 9 h after suxibuzone administration and exceeded IC50 of PGE2 for at least this time. In the present study, suxibuzone was well tolerated following i.v. injection.

Evaluation of dosage forms. VI. Studies of commercial oxyphenbutazone tablet dosage forms.

Dissolution-dialysis studies of commercial tablets of oxyphenbutazone were carried out to establish the applicability of this technique for the in vitro evaluation of oxyphenbutazone dosage form. While disintegration time and dissolution rate studies did not give a true indication of bioavailability, an excellent correlation was obtained between the dialysis rate constant K and the pharmacokinetic parameters AUC and Cmax.

Pharmacokinetics of phenylbutazone in camels.

OBJECTIVE: To document disposition variables of phenylbutazone and its metabolite, oxyphenbutazone, in camels (Camelus dromedarius) after single i.v. bolus administration of phenylbutazone, with a view to making recommendation on avoiding violative residues in racing camels. ANIMALS: 6 healthy camels (4 males, 2 females), 5 to 7 years old, and weighing from 350 to 450 kg. PROCEDURE: Blood samples were collected to 0, 5, 10, 15, 45, and 60 minutes and at 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 8, 12, 24, 26, 28, 30, 40, 48, 50, 53, and 60 hours after i.v. administration of 4.5 mg of phenylbutazone per kg of body weight. Urine was obtained in fractions during the entire blood sample collection period. Serum and urine phenylbutazone concentrations were measured by high-performance liquid chromatography; assay sensitivity was 100 ng/ml. Serum oxyphenbutazone concentration was measured by gas chromatography/mass spectrometry; assay sensitivity was 10 ng/ml. RESULTS: Disposition of phenylbutazone was best described by a two-compartment open model. Mean +/- SEM elimination half-life was 13.44 +/- 0.44 hours. Total body clearance was 12.63 +/- 1.64 mg/kg/h. Renal clearance was between 0.3 and 0.4% of total body clearance. The elimination half-life of oxyphenbutazone was 23.9 +/- 2.09 hours. CONCLUSIONS: The elimination half-life and total body clearance of phenylbutazone in camels are intermediate between reported values in horses and cattle. Extrapolation of a dosage regimen from either species to camels is, therefore, not appropriate. Elimination of phenylbutazone in camels is mainly via metabolism. Owing to the long half-life of phenylbutazone and of oxyphenbutazone, and to the zero drug concentration regulation adopted by the racing commissioner in the United Arab Emirates, practicing veterinarians would be advised not to use phenylbutazone in camels for at least 7 days prior to racing.

Pharmacokinetic and pharmacodynamic studies on phenylbutazone and oxyphenbutazone in goats.

Phenylbutazone was administered intravenously and orally to six goats as a single dose of 4.4 mg/kg and its disposition and bioavailability and the disposition of its active metabolite, oxyphenbutazone, in plasma were investigated. The effect of the administration of the drug of oxyphenbutazone on ex vivo serum thromboxane (TX)B2 generation in platelets was also studied. Phenylbutazone was eliminated slowly with mean (se) elimination half-lives (t1/2 beta) of 15.3 (1.15) hours and 22.0 (3.32) hours after intravenous and oral administration, respectively. The bioavailability of phenylbutazone paste administered orally was 61 (7) per cent (corrected by the t1/2 beta) and relatively slow absorption was observed, as indicated by a time of maximum drug concentration (tmax) of 3.47 (0.39) hours and a mean absorption time (MAT) of 10.4 (8.61) hours. The concentration of oxyphenbutazone in plasma was low and the ratio of the areas under the curve (AUC) of oxyphenbutazone to phenylbutazone was approximately 0.02:1 after both intravenous and oral administration. Thromboxane B2 generation in the platelets was significantly inhibited (P < 0.05) from one to 12 hours after intravenous administration and from two to 12 hours after oral administration. The results suggest that phenylbutazone is a potentially useful non-steroidal anti-inflammatory drug for use in goats by either route of administration.

Comparative pharmacokinetics of phenylbutazone and its metabolite oxyphenbutazone in clinically normal horses and donkeys.

OBJECTIVE: To compare plasma disposition of phenylbutazone and its metabolite oxyphenbutazone after i.v. administration of phenylbutazone in horses and donkeys. ANIMALS: 4 clinically normal horses and 6 clinically normal donkeys. PROCEDURE: Blood samples were collected from each animal at time 0 (before) and 5, 10, 20, 30, 45, 60, 90, 120, 180, 240, 300, 360, and 480 minutes after i.v. administration of a bolus dose of phenylbutazone. Serum was analyzed in triplicate by use of high-performance liquid chromatography for determination of phenylbutazone and oxyphenbutazone concentrations. The serum concentration-time curve for each horse and donkey was analyzed separately to estimate model-independent pharmacokinetic variables. RESULTS: Significant differences were found in several pharmacokinetic variables of phenylbutazone and oxyphenbutazone in horses, compared with donkeys. Mean total body clearance of phenylbutazone in horses was fivefold less than that in donkeys (29.3 and 170.3 ml/kg/h, respectively). Mean values for area under the curve and mean residence time in horses (118.3 micrograms/h/ml and 3.6 hours, respectively) were significantly greater than values in donkeys (28.3 micrograms/h/ml and 1.7 hours, respectively). Mean values for apparent volume of distribution at steady state were not significantly different between horses and donkeys. For oxyphenbutazone, mean time to peak concentration in donkeys was significantly less than that in horses (1.6 and 6.4 hours, respectively). CONCLUSION: Phenylbutazone clearance in donkeys was higher than that in horses, and appearance of the metabolite oxyphenbutazone in serum was more rapid in donkeys than in horses, indicating that hepatic metabolism of phenylbutazone is more rapid in donkeys than in horses. CLINICAL RELEVANCE: Because serum concentration of phenylbutazone after single i.v. bolus administration (4.4 mg/kg of body weight) decreases more rapidly in donkeys, compared with horses, phenylbutazone may require more frequent administration in donkeys to achieve therapeutic efficacy.

The disposition of suxibuzone in the horse.

A high performance liquid chromatographic method is described to determine the anti-inflammatory drug suxibuzone (SXB) and its major metabolites phenylbutazone (PBZ) and oxyphenbutazone (OPBZ) in equine plasma and urine. When suxibuzone (6 mg/kg) was administered intravenously (i.v.) or orally (p.o.) no parent drug was detected in plasma or in urine. The disposition of the metabolite PBZ (i.v.) could be described by a 2 compartment model with a beta half-life varying from 7.40 to 8.35 h. Due to severe side effects the use of i.v. suxibuzone should not be encouraged in the horse. PBZ and OPBZ were detected in plasma and urine after p.o. SXB administration. Peak plasma PBZ concentrations (8.8 +/- 3.0 micrograms/ml) occurred 6 h after oral dosing and the terminal exponential constant was 0.11 +/- 0.01 h-1. Phenylbutazone and oxyphenbutazone were detectable in urine (> 1 microgram/ml) for at least 36 h, after p.o. administration. SXB was not hydrolyzed in vitro by horse plasma. Equine liver homogenates however appeared to have a very high capacity for hydrolysing SXB, indicating that first-pass effect could be responsible for the rapid disappearance of this NSAID in the horse.

A new approach to encapsulating nonsteroidal anti-inflammatory drugs. IV. Effect of cellulose derivatives with different functional groups on the bioavailability and gastric ulcerogenic activity of acidic as well as basic nonsteroidal anti-inflammatory drugs.

The bioavailability and gastric ulcerogenic activity of oxyphenbutazone and glafenine (acidic and basic nonsteroidal anti-inflammatory drugs), coated with different cellulose derivatives were assessed in albino rats. The cellulose derivatives chosen have different functional groups, acidic (carboxymethyl cellulose), basic (chitosan) and neutral (hydroxypropylmethyl cellulose). The bioavailability was dependent on the drug and polymers. Generally, all the cellulose derivatives chosen decreased the gastric ulcerogenic activity of the drugs studied.